In recent years, there has been an increasing demand for electrically-driven actuation of valves, particularly for those valves controlling heating media in rapid-response water heaters. Electric valve actuators have many advantages over pneumatic or hydraulic valve actuators. Specifically, electrically actuated valves consume small amounts of ordinary electric energy, as opposed to costly compressed air or hydraulic pressure. Also, precision, strength and reliability are equal to or better than alternative operating media, and their electronic circuitry can interact directly with computerized building controls.
A disadvantage associated with electrically-driven actuators, particularly those with close ratio (high-speed) gear trains, is that most do not maintain a sufficient holding torque when de-energized. The problem lies with the rack-and-pinion linkage systems, which are generally used in conjunction with electric valve actuators. For more information regarding the use of rack-and-pinion linkage components with valve actuators see, for example, U.S. Pat. No. 4,597,556 which is incorporated herein by reference in its entirety. As a general rule, these linkage systems can transmit force equally well in both directions, which can allow pressure from within a valve to move the gear-train, including the rack-and-pinion linkage, backward reducing the amount of seating force applied to the valve. The reduction in seating force can allow undesired flow through the valve to occur. In the case of rapid response water heaters, this undesired flow can allow overheating of the hot water supply, which can lead to scalding.
The current solution to this problem has been to increase the stroke length of the actuator linkage which reduces the mechanical advantage imparted by the rack-and-pinion linkage. The benefit being that although it would require more force to close the valve, it would also require more pressure from within the valve to force the gear-train of rack-and-pinion linkage backward, re-opening the valve when the electric actuator is de-energized. Similarly, the size of the electric drive motor used to operate the valve actuator can also be increased, increasing the inertial mass of the actuator, which further increases the force required to drive the gear train, including the electric motor, backwards. However, these measures are not ideal solutions to the problem because both are only partially effective at preventing unwanted valve opening. Additionally, increasing the stroke length of the actuator linkage and increasing the size of the electric motor will decrease the efficiency and compactness of the valve actuator.
What is needed is a valve linkage apparatus capable of producing a large output force from a small input torque while also being capable of precisely positioning the internal control elements of a valve. Furthermore, the valve linkage apparatus would preferably be able to maintain a valve in its closed position, with full seating force applied, even when all external sources of operating energy are unavailable to the valve actuator.